Sailing
Sailing between two positions on the earth's surface involves calculating the course and distance between them.
The shortest distance between any two points on the earth is the shortest arc of the great circle through those points.The great circle track crosses the various meridians at different angles.
A rhumb line is a line on the earth's surface crossing all meridians the same angles.
Departure
The departure between two places is the East West distance between them in nautical mile.
PARALLEL SAILING
When the starting and destination positions are on the same latitude the ship could sail along a rhumb line due to East or West along the parallel of latitude of two places.Sailing in this manner is therefore called parallel sailing.
Since the distance traveled is East or West it is equal to the departure between two positions.
PLANE SAILING
Plane sailing is sailing along a rhumb line one place to another,which are not situated on the same latitude.Small portion of the earth's surface may be considered to be flat or plane surface without applicable error.Where the distance between two place is less than 600 miles also plane trigonometry can be used to obtained few navigational formulas.
